Electron discharge device



P. P. DERBY ELEGTRON DISCHARGE DEVICE Mai'ch 8, 19 49.

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Filed March 10, 1945 Patented Mar. 8, 1949 UNITED STATES PATENT OFFKIE ELECTRON DISCHARGE DEVICE Palmer P. Derby, Boston, Mass, assignor to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application March 10, 1945, Serial No. 582,046

8 Claims. 1

This invention relates to a magnetron microwave oscillator.

An object of this invention is to produce such an oscillator which can operate successfully with high power output and efficiency on continuous wave operation.

Another object is to devise such an oscillator in which the separation between the principal mode and the closest adjacent mode is greater than a critical value below which successful continuous wave operation does not occur.

A further object is to devise such a magnetron in which the cathode structure is arranged to conduct away large amounts of heat generated at the cathode during continuous wave operation, said conduction to occur at a rate sufficient to prevent the overheating of the cathode.

The foregoing and other objects of this invention will be best understood from the following description of an exemplification thereof, reference being had to the accompanying drawings wherein:

Fig. l is a vertical section through one form of my invention;

Fig. 2 is a horizontal section taken along line 2-4 of Fig. 1;

Fig. 3 is a broken-away perspective view showing two of the anode paddles; and

Fig. 4 is a set of curves showing in a qualitative manner certain relationships within the magnetron which may be taken into consideration in embodying the principles of my invention in a practical device.

The magnetron illustrated inFigs. 1-3 comprises a tubular anode structure I made of a cylinder of conducting material, such as copper. A plurality of radially-disposed plates 2 likewise formed of conducting material, such as copper, are soldered in place along the inner surface of the hollow cylinder I. Each pair of plates, together with the intervening portion of the cylinder I, define a cavity resonator. The inner ends of the radial plates are adapted to serve as anode faces for receiving electrons emitted from the centrally-disposed cathode 3. Alternate plates 2 are electrically connected bymeans of conducting straps 4, 5, 6 and l. The straps 4 and 5 are located at one end of the anode structure, the strap 4 being electrically connected to alternate plates 2 and the strap 5 connected to the intervening alternate plates. At the other end of the anode structure, the strap 6 is connected to those plates to which the strap 5 is connected, and the strap 1 is connected to those plates to which the strap 4 is connected. Such a strap arrangement substantially decreases the tendency for the device to operate in various It is particularly effective in tional hollow conductor, not shown, may be connected to the pipe 9 so as to surround the con ductor 8', forming therewith a concentric transmission line through which the energy generated by the oscillator may be fed.

The upper and lower ends of the cylinder I are closed by the upper and lower caps II and I2 formed of a suitable conducting material, such as copper, and hermetically soldered into the ends of the cylinder l. The upper cap H is provided with a central opening into which a magnetic pole piece 13 is hermetically soldered so as to close said opening. Similarly the lower cap 12 is provided with a central opening in which is hermetically soldered a lower pole piece M. The pole pieces l3 and M are formed of highly permeable iron. Any suitable form of external magnet may be applied to these pole pieces so as to impart opposite polarities thereto, whereupon the proper magnetic field is 'set up within the magnetron v about the cathode 3.

The cathode 3 is formed with a central tubular section l5, the outer surface of which is coated with firmly adherent electron-emitting oxides. Connected to the upper and lower ends, respectively, of the central section I5 are enlarged hollow end sections l6 and I1. The entire assembly l5, I6 and Il may be made of a suitable metal, such as nickel. A heating coil [8 is supported One end IQ of the coil" within the section l5.

I8 is welded within and to the hollow section l6, and the lower end 20 extends through and beyond the lower section [1. The lower end 20 of the heater is in turn welded to a lead-in conductor 2!. The pole pieces [3 and M are provided, respectively, with bores 22 and 23, into which are received the enlarged end sections it and ll of the cathode 3. The end section I! is provided with .a reduced extension 24 which fits into the upper end of a hollow rod 25 made of a highly' conductive material, such as copper.

The other end 1 The rod is soldered in the lower end of the hollow rod 255- Supported on the outside of the lowerendof said rod 25 is a heat radiator 32 provided with a plurality of heat-radiating fins-.332

When a magnetron of the ty-pe. as described: herein is operated on continuous wave, a veryconsiderable amount of heat is. fed-back into the cathode by electronic discharge occurring between the cathode and the anode. In order for such operation to be successful, this heat must be conducted away-fromthe emitting portion of the cathode at a sufiicient rate. to prevent overheating of: the cathode. The-arrangement which I have describedabove accomplishes this result. It will be noted that the end sections I6 and I I of the cathode. consist of relatively massive metal bodies having a high heat conductivity and a relatively high heat capacity as compared with the emitting section: l; Also the end sections I6 and I1 extendfor a considerable distance within the massivemetal bodies provided by the polepieces l3 and I4; Heat is. thus readily radiated from the end.- sections I6 and I! to the massive bodies l3 and M from which the heat may be conducted very rapidly. If desired the bores 22 and 23'withinthe pole pieces l3 and Himay be provided with black surfaces so. as to more readily absorb the energy radiated from the endsections l6 and I1. Also the lower section IT- is in intimate contact with the highly conducting: rod 25 so that in addition. to the radiation the heat-is led away from the: end section I! by conduction through the. rod 25.. The heat radiator 32 forms an excellent means-forv dissipating this heat. In. some. instances it may be desired to blow air against the. heat radiator. 32 to conduct the; heat away more: rapidly.

Ihavefound that with such a construction, extraordinaryamounts of power may be delivered by the tube as a continuous Wave oscillator without overheating the cathode.

Eyenlif a magnetronis constructed sothat the heat is conducted away from the cathode with suificient rapidity to-prevent overheating, nevertheless successful continuous Wave operation ofthe magnetron may not be secured unless additionalfeatures are incorporated in the tube. For effective continuous Wave operation, the magnetron should be operated with a relatively high ratio between the voltage between the anode and cathodeandlthe average current flow between the anodeand cathode. I have found that a ratio of the order of1,0'00'volts/amp. or greateris desirable. This is a dynamic ratio and not a static one. The ratio represents the change in anode voltage required to produce a desired change in anode. current. However, when an attempt is made to operate prior types of micro-Wave magnetrons on continuous wave, the tube tends to oscillate at various frequencies other than the desired frequency,v and thereforeis substantially inoperative for continuous wave purposes. I have found'th'at't'his defect can be eliminated-if theper'centagemode separation in themagnetron is:made gre'at'er'than' a criticalvalue, dependingon the' mode index number;

In a magnetron, the mode index number is an expression of the number of voltage maxima points which exist throughout the tube structure during the generation of oscillations. In a multiple anode multiple cavity resonator type magnetron, such as that illustrated herein, it is desirable to concentrate the oscillations produced in the. principal mode in which successive anode electron-receiving" faces are of opposite voltage phase. If N represents the number of anode plates 2 within the tube, and n represents the indexnumber of the principal mode, then In amagnetronrsuch as I have been discussing above, the percentage mode separation between the principal mode and the next lower mode is where Kn i'sthe Wave length ofthe'principal mode and \n-1 is the wave length. of the next lower index numbermode at which the. tube structure has'a' substantial tendency to oscillate. Likewise the percentage mode' separation between the principal mode and the next highermode' is (Equation 1) (Equation 2) ndi u- A where /\n+l isthewave length of the-next higher (Equation 3) index number modeat which the tube structure (Equation 4) higher. or lower index numb'er'whichis closest to:

the principalimode and isthewell-known symbolfor' the'absolute magnitude of the difference between two quantities. I have-"found that for successful continuous wave operation, the percentage mode-separation must be" grea-ter'than' a critical value which is equal to ig!+ 1 (Equation 5) cal percentage of mode separation against thenumber of anode arms.

From the foregoing it will be noted that as the number" of" anode 'arms' increases, the permissible mode-separation decreases so that by' choosing a large number of anode arms; suchas" twenty or more, continuous Wave operation might be ob-- tain'ed witha percentage mode separation of about 9.1% or'l'ess.

However, the-use of such a large number ofanodearms-introdu'ces various difficulties so thatI preferto utilize a smaller number of" anode arms, such as eighteen or less;

I have also found that if certain relationships are maintained in a tube, the desired critical mode separation may readily be reached and in most instances greatly exceeded. For all practical purposes the percentage mode separation must be substantially greater than the critical value, due to the following reasons. When a magnetron is set up with a certain magnetic field and the anode voltage is increased to a value at which the anode-cathode current begins to flow, the tube will start oscillating, and if the mode separation is greater than the critical value, these, oscillations will be in the principal mode. However, if the mode separation is only slightly greater than the critical value, the anode-cathode current can be raised only a slight amount before additional modes are excited, thus rendering the tube unfit for continuous wave work. As the mode separation is increased beyond the critical value, the greater may be the increase or excursion of the anode-cathode current. It will be seen that the amount of power which such a tube can deliver is dependent on the magnitude of the anode-cathode current excursion. Thus in a practical tube the mode separation should be substantially greater than the critical value. Thus, for example, in a tube having sixteen anode arms, the critical percentage mode separation is about 11%. Yet for practical purposes I prefer to construct the tube with a percentage mode separation of the order of 50% or greater.

The factors which determine the percentage mode separation can be determined from the following empirical relationship which is approximately true for all practical configurations:

(Equation 6) where w=21rf; i being the frequency generated by the tube at the particular mode under consideration;

L=the total lumped inductance of one cavity resonator between two of the arms 2;

C=the total lumped capacity of said one cavity resonator. This includes the added capacity introduced into the resonator by the adjacent straps 4-4;

Cs=the capacity of the straps :between the anode arms defining said one cavity resonator;

n being the index number of the mode being considered.

A=LS+M5; LS being the self-inductance of that part of one strap extending between the two anode arms of said one cavity .and Ms being the mutual inductance between those portions of two adjacent straps which extend between said two anode arms; and

The absolute magnitudes of Cs and A are usually made so small that the expression l and thus cot 0 is zero. Under these conditions 6 the formula for the principal mode can be reduced substantially to the form l o (Equation 7) The factors which vary as 0 changes from the principal mode I. 2 to an adjacent mode are sin 0 m=1+cot 0 by adjusting the scales as indicated, the same dotted curve may serve to indicate the values of both factors. It will be noted that the effect of the expression in Equation 6 containing sin 0 is opposite to that of the expression containing cot 0 upon the mode separation. However, the proportionate effect of the sin 6 factor is less and also, as above indicated, the expression containing this is usually made so small as to be insignificant. Thus for most purposes we can consider only the effect of the expression containing cot 0.

If we now select a value of w for the next mode adjacent to the principal mode to satisfy the requirements for continuous wave operation described above, we can satisfy the requirements of Equation 6 by properly selecting the relative proportions of L, C, Cs, Ls and Ms. We see, for example, that larger values of L will increase the mode separation. Therefore in my embodiment I have constructed my cavity resonators to have a relatively large cross-sectional area. Also if we make MS larger and Ls smaller, the mode separation becomes larger. Thus it becomes desirable to place the straps 4 and 5, and 6 and 1, closely adjacent to each other to obtain a tight coupling between them.

If for some reason the mutual inductance of the straps cannot be made sufiiciently large to give a large separation of modes, then the factor in Equation 6 containing sin 0 can be made to predominate and give the desired mode separation. This can be done by making the strap capacity Cs larger. Such large strap capacity may :be obtained by increasing the areas of the straps facing each other.

i-aByrfthetuse-rof" the storm strap; and .straps structuresurrounding'said cathode and comprisin the specification-pendiclaims, 1.1 ,do: notaintend ing a, plurality of anode arms disposed about said to be limited to the shape of the straps 41 as cathode, each armhaving an electron-receiving 'illustratedherein. Any cross-sectional shape for face, and each pair of adjacent arms,:together the conductors interconnecting the anode arms is 5 wit-h that portion of said anode structure lying included in'these 'termsprov'ided the'values C5, therebetween, definin a cavit resonator, alter- Ls and Ms are such as to give the-mode sepahate-anode arms being directly interconnected by ration as defined. avstrap, the intervening anode arms. being -.di-

For purposes of clarity in the claims, Equation rectly interconnected by anotherstrap adjacent 6 for the two values An and Aa-canbe expressed as said first-mentioned strap, said magnetron. hav- 21w) 1 2L 1 1 2 (Equation 8) I: -i( c.(L.+M. -1)] and 21w 1 2L f 1 LC 1 Tin-Hi1s 1 21117 v N (Equatlm 9) T OI(L+MI) 1 IE-l H :c0t N 7r 1 2 sin 2 1rwherevis the velocity of light. .25 ing a percentage mode separation substantially If a tube is constructed to scale as illustrated greater thanthe critical value in the drawings, then such a tube will have in- 1 corporated. in it the principles of my invention and will operate successfully on continuous wave. 1

It is to be understood that this invention is not .30 limited to the .particular details as described WhereN is the number of anode arms. above. Although the use of straps is an excellent 4. A magnetron Compr si a cathode, an anode way of securing. the requisite mode separation, yet ct -S u di g said cathode and comprisdt i possibletobuil u t ped agnetrons with .ing a plurality of anode arms disposed about said mode separations which exceed the critical value ,35 cathode, each arm having an electron-receiving of this invention. Aslong assuch separation .exface, and each pairof a j ent arms cons itutin ceeds this critical value in the manner as dethe .side walls of a cavity re n or. alternate scribed above, successful. continuous wave operaod arms being t y interconnected by a tion can be secured. Many other equivalents will strapnthe intervening anode arms being directly :suggest th m fl t those skilled in the :art..,-4o interconnected by another strap, the ratio between Itis accordingly desired that the appended claims the 511m of the Self-inductance 0f t e portion o be given a broad interpretation commensurate one of said straps between two of said anodearms with the scope of the invention within the art. and the mutual inductance betweenthe portions wh t is claimed oftwo of said'straps between said two anode arms .1, A magnetron comprisin a, cathode, ananode 4 and the difference between said Self-inductance structure .surrounding said. cathode andcomprisandsaid mutual inductance being la ge compared ing. a pluralityof anode arms disposed about-said t0 e D Od Ct.

-cathode, each armhavin an electron-receiving Q A face, and each pair ofadjacentarms, .together with that portion of. said anodestructure lying .therebetween, defining a :cavity resonator, said where w=21rf, ,1 being the frequency of the magmagnetron. having a percentage mode separation netronat' the substantially greater .than thev critical -:value whereN is thenumber. of anode arms.

.face; and each. pair of adjacent arms, together ctherebetween, definin a cavity resonator,.alter- 5 1 .N 1 2 .mode, N being the number of said anode arms,

s=the capacity of the straps between tWo adjacent-arms, and.A=Ls+Ms, Ls being said selfinductance and Ms being said mutual inductance.

5. .A magnetron comprising a cathode, an. anode structure surrounding said cathode and comprising a plurality of anodearms disposed about said cathode, each arm having an electron-receiving face, and each pair of adjacent arms constituting the side walls of a cavity resonator, alternate anode arms .being directly interconnected by a strap, the intervening anode arms being directly interconnected by another strap, L, C, Ls, Cs and Ms being related in the equations "2. A magnetron comprising a cathode, an anode "structure surrounding said cathode and comprising a plurality of anode arms disposed about said cathode, each .arm having an electron-receiving "with that portion of said anode structurelying nate anode arms being directly interconnected by "a strap, the intervening anode arms being direct- -ly interconnected by another strap, said magnetron having a percentage mode separation substantially greater than the critical value c 1 L +M 1 z 2 'where N;isthenumber of=anode arms. a 8 l 0 (L H v vA I 8 v 3. A magnetron comprising a-cathodaan anode I n I a and so that is substantially greater than the critical value l {V 1 2 the above symbols having the following values:

6. A magnetron comprising a cathode, an anode structure surrounding said cathode and comprising a plurality of anode arms disposed about said cathode, each arm having an electron-receiving face, and each pair of adjacent arms constituting the side walls of a cavity resonator, said cathode comprising a central electron emitting portion of relatively low heat capacity, end sections of metal and of relatively high heat capacity connected to the ends of said central portion and in good heat conducting relation therewith, two pole pieces supported at opposite ends of said anode structure, said pole pieces being of magnetic material and of relatively massive size relative to said end sections, said pole pieces being provided with elongated bores, said end sections projecting into said bores, each for a substantial distance.

7. A magnetron comprising a cathode, an anode structure surrounding said cathode and comprising a plurality of anode arms disposed about said cathode, each arm having an electron-receiving face, and each pair of adjacent arms constitutin the side walls of a cavity resonator, said cathode comprising a central electron emitting portion of Number relatively low heat capacity, an end section of metal and of relatively high heat capacity connected to one end of said central portion and in good heat conducting relation therewith, a pole piece supported at one end of said anode structure, said pole piece being of magnetic material and of relatively massive size relative to said end section, said pole piece being provided with an elongated bore, said end section projecting into said bore for a substantial distance, the opposite end of said central portion being in good heat conducting relation with a highly heat conductive rod, said rod extending exteriorly of said anode structure and carrying a heat radiator.

8. A magnetron comprising a cathode, an anode structure surrounding said cathode and comprising a plurality of anode arms disposed about said cathode, each arm having an electron-receiving face, and each pair of adjacent arms constituting the side walls of a cavity resonator, said cathode comprising a central electron emitting portion of relatively low heat capacity, end sections of metal and of relatively high heat capacity connected to the ends of said central portion and in good heat conducting relation therewith, two pole pieces supported at opposite ends of said anode structure, said pole pieces being of magnetic material and of relatively massive size relative to said end sections, said pole pieces being provided with elongated bores, said end sections projecting into said bores, each for a substantial distance, one of said end sections being in good heat conducting relation with a highly heat conductive rod, said rod extending exteriorly of said anode structure and carrying a heat radiator.

PALMER P. DERBY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Samuel Dec. 8, 1936 Linder May 16, 1944 McArthur Dec. 17, 1946 Spencer Mar. 18, 1947 FOREIGN PATENTS Country Date Great Britain July 11, 1939 Number 

